Generally, petroleum, coal and/or natural gas are burnt as heat sources for generation of steam and heating water in generating station and factories.
Fuel gases such as city gas and propane gas have been generally and widely used as heat sources for cooking.
Griddles for use in the restaurant business, such as where steaks and Japanese style tacos (Okonomiyaki) are served, commonly employ fuel gases as a heat source. These griddles for use in the restaurant business have thick iron cooking plates. Gas is burnt under these plates so as to heat up these plates, and materials to be cooked, such as meat, are placed on the surface of the plate and heated with the conduction heat coming through these plates.
Other methods for heating iron cooking plates are known. One method is where an iron cooking plate is heated with the use of a nichrome resistance wire heater. Another method is where an iron cooking plate is indirectly heated by an induction heat source as disclosed in Japanese Examined Patent Application No. 58-39525 and Japanese Unexamined Utility Model Application No. 56-86789.
Electrical resistance heater are preferred as a heat source in particular industrial production processes where heating by burning petroleum, coal and/or natural gas is not the preferable method for heating. An example is the process of combining an axle with a wheel and an inner bearing race of an axle with a bearing for rolling stock or an electric car. In these cases, a round metal plate with a central round hole must be combined with an axle. This is done by heating the round metal plate with a central round hole so that the central round hole is thermally expanded. Then the axle is inserted into the central round hole, and the plate is cooled down to give a tightly combined assembly. Electric heating is the most suitable method for heating in these types of cases.
The most frequently used method of heating in the restaurant business is gas burning, as discussed above, which results in the problem of management of combustion gas evolved. The combustion gas should not be exhausted towards the customers. And, in cases where the combustion gas is exhausted in a direction other than toward the customers, for example, beneath or to the side of the cooking plate, the efficiency of heating is considerably decreased. Therefore, the combustion gas is currently exhausted towards the cook. When fuel gas is used, an exhaust duct must be provided, and this is constructed above the cook's head. Therefore, the cook has to breathe in the combustion gas and oily smoke, and is exposed to high temperature and high humidity, leading to a terrible working environment.
A further problem in heating by gas burning is that the oily smoke dirties walls and ceilings.
Another problem in heating by gas burning is that there exists the dangerous possibility of a gas-explosion or an oxygen deficiency. Therefore, at places such as an underground market or a high building, the use of gas burning is often restricted for safety reasons.
A further problem in heating by gas burning is that the cooking plate suffers an irreversible deformation due to strain inside the plate caused by thermal stress.
Also, heating with a nichrome resistance wire heater presents a serious problem in that the cooking plate is destructively deformed by the large strain caused by the thermal stress due to the local heating and the increased electrical capacity. Therefore, the method of electrical heating with a nichrome resistance wire heater presents difficulties.
Additionally, in known methods of indirect heating with an induction heat source there exists a critical and unavoidable problem. Electromagnetic vibration of the plate occurs, resulting in loud noises, and sometimes destruction of the cooking apparatus itself.